1. Field of the Invention
The present invention relates to the qualitative and quantitative determination of chemical agents. More particularly, the present invention is a method for determining O-ethyl S-(2-diisopropylanimoethyl) methylphosphonothiolate, better known as VX, vapor. Most particularly, this invention permits the generation of a purer non-contaminated VX vapor and the analytical determination of the VX samples collected under various relative humidity conditions.
2. Brief Description of the Related Art
O-ethyl S-(2-diisopropylanimoethyl) methylphosphonothiolate, better known as VX, is an extremely toxic chemical warfare agent. It is a relatively stable compound that undergoes slow degradation. For example, the non-stabilized VX of 95% purity, without adding a compound stabilizer, decomposes at a rate of approximately 5% per month at 25.degree. C. The decomposition impurities include ethyl methylphosphonic acid, methylphosphinic acid, diisopropylaminoethyl mercaptan, diethyl methylphosphonate, and ethanol.
As an extremely toxic compound, VX detection is best accomplished at the lowest possible concentration. Chemical detectors used for the detection of VX require calibration. Vapor samples of a known concentration of VX are used to test and evaluate these detectors. The VX samples need to be qualitatively pure and quantitatively analyzed to be useful in properly evaluating different chemical agent detectors.
Pure VX samples, of 95% or greater (CASARM quality), are useful in testing chemical agent detectors. CASARM (Chemical Agent Standard Analytical Reference Materiel) VX is used for instrument calibration to provide truest reference. Compound stabilizer is not added within the neat VX sample. However impurities within the VX sample itself, such as at approximately 5%, affect detector testing in a disproportionate manner. Volatility differences between VX and accompanying impurities within a sample produce vapors that are not representative of the respective amounts of VX and impurities within the liquid sample. Greater vaporization rates of the impurities produce a vapor saturated with impure vapors during the initial vaporization of the VX sample. Impurities contained in a VX sample which are more volatile than VX, interfere with most chemical detectors as well as most analytical methodologies. As the amount of impurities within the VX sample decreases, an increase in the reliability of the testing of chemical agent detectors occurs. For example, 95% pure VX samples, having impurities such as diisopropylaminoethyl mercaptan (thiolamine) at 5%, produce vapors containing less than 1% VX vapor and greater than 99% thiolamine vapor. As such, these impurities cause significant interference with testing most chemical detectors as well as most analytical methodologies. As discussed above, although an impurity may only exist in small percentage in the liquid VX sample, it may become the major constituent in the vapor mixture when vaporized. For proper VX vapor evaluation of detection devices and analysis, elimination of non-VX impurities (such as thiolamine) is necessary.
Several methods have been used to quantify a VX vapor sample. The Shoenemann reaction determines VX concentration by converting the VX into a G-agent analog before reacting with indole and peroxide to produce a fluorescence reaction. However, the reaction is interfered with by the existence of excess thiolamine. The thiolamine interferes with precipitate filtration as well as affecting the fluorescence reading of the sample. Thus, thiolamine may cause fluorescence false readings that affect the actual VX vapor concentration.
The presence of thiolamine also affects the outcome of the enzymatic method of analysis, which was developed to substantially negate the thiolamine effect. The lengthy enzymatic method requires detailed knowledge in the use and maintenance of the complex equipment. The enzymatic method also results in the generation of significant amounts of waste solution, requiring extensive disposal procedures. Additionally, the enzyme method does not distinguish the effects from other impurities in the VX vapor sample, nor identify the relative abundance of the impurities present.
Analysis by a gas chromatographic method, using a solvent containing bubbler to collect a vapor sample, provides a separation identification of the impurity vapors from the VX vapors. The method draws and collects the vapor into a solvent, only a small portion of which (approximately 5 microliters of a 5 milliliter sample) is then injected into a gas chromatograph (GC) equipped with a flame photometric detector (FPD). The VX peak, separated from the impurity peaks, is used to determine the VX vapor concentration. Because of the injection limitations, the gas chromatographic method has limited detection capability of approximately 0.04 mg/m.sup.3 of VX concentration or greater. The accuracy and reliability of the method are compromised from the combined affects of factors such as long sampling times of five to ten minutes, collection efficiency, solvent evaporation, and small sample injection. The method, although effective for analyzing VX vapor at low relative humidity conditions, is unable to analyze VX vapor sample collected under high humidity conditions. When used under high humidity conditions, the VX peak may be lost or substantially diminished from an expected concentration.
Also known in the art, VX vapor may be analyzed using the Miniaturized Continuous Air Monitoring System (MINICAMS), described in U.S. Pat. No. 5,014,541 (Sides et al.) and U.S. Pat. No. 5,052,805 (Sides), which issued on May 14, 1991 and Oct. 1, 1991, respectively. It provides a method of detecting VX vapor by passing the VX vapor through a conversion pad that converts the VX vapor into a more volatile G-analog similar to the conversion process of the fluorescence method, described above. Conversion efficiencies are affected by the age of the conversion pad, the temperature and humidity, the flow rate through the pad, impurities and other factors. The MINICAMS' FPD detector detects the converted G agent analog peak, thereby deriving an equivalent VX concentration. Such MINICAMS method is not useful for precise quantitative and qualitative analysis because of the quality of the conversion pads and the unknown effects from the associated impurity vapors. Additionally, such MINICAMS method lacks the ability to separate or distinguish the relationship between the impurities and VX.
In addition to low volatility, VX readily adheres to most surfaces contacted. As such, low concentrations of VX sample are significantly reduced when transferred through any additional tubing or connections. This reduction significantly interferes with conventional methods in quantifying VX agent vapors for testing chemical agent detectors.
In view of the foregoing, improvements in chemical monitoring have been desired. It has been desired to provide an improved methodology for sampling VX both quantitatively and qualitatively.